Environmental stress cracking of PVC and PVC-CPE

The craze initiation of PVC and PVC-CPE in a number of vapour and liquid environments was studied. The observed decrease in the craze-initiation stress could be ascribed to the absorption of components by the PVC and the PVC-CPE matrix. The magnitude of the absorption depends on the activity of the components and on their interaction with the PVC matrix. The activity of the components was defined according to thermodynamics. The interaction of the components can be obtained from measurements of the angle of contact, though it is preferred to quantify this interaction by absorption experiments. A simple model is presented to predict the long-term craze-initiation stress of PVC in environments.     

Environmental stress cracking of PVC and PVC-CPE

The fracture toughness of Polyvinylchloride (PVC) and PVC modified with 10% chlorinated polyethylene (PVC-CPE) was studied in vapour and in liquid environments by crack growth measurements on single-edge notch specimens under three-point bending at 23°C. In addition, some results obtained in air at lower temperatures are presented. The fracture toughness is quantified by a stress intensity factor leading to failure after a given loading period. It is shown that for a given slow crack growth rate at 23 °C, the environment hardly affects the fracture toughness of PVC. In contrast, the slow crack growth in PVC-CPE at 23 °C is accelerated by the presence of benzene vapour, n-octane/benzene mixtures and gas condensate. A decrease in temperature results in an increase in fracture toughness, both for PVC and for PVC-CPE. A Dugdale model to describe the craze ahead of the crack was used to analyse the observed changes in fracture toughness.     

Environmental stress cracking of PVC: Effects of natural gas with different amounts of benzene

Craze initiation, craze growth and ultimate fracture were studied in PVC (Polyvinylchloride) and PVC-CPE (PVC blend with 10% chlorinated polyethylene) under a constant load in air and in natural gas enriched with benzene. The craze initiation results are similar for PVC and PVC-CPE. The craze initiation stress is decreased dramatically at high benzene concentrations ( 25000 p.p.m.). Preferential and enhanced sorption of benzene molecules near surface inhomogeneities, the craze initiators, are held responsible for this phenomenon. Initial crazegrowth rates in natural gas enriched with benzene increase compared to those in air. However, in all the environments studied, a limited logarithmic craze growth is observed, and growth seems limited to this constant logarithmic interval. The logarithmic craze growth and the termination of craze growth are attributed to a reduction of the stress at the craze tip. The failure mode of PVC and PVC-CPE in air and in gas with 5000–6000 p.p.m. benzene is yielding. A brittle branch appears in the failure curve (failure stress against loading time) in the more concentrated benzene vapours. This branch is attributed to a reduction in the stability of the crazes.     

Environmental stress cracking of polycarbonate catheter connectors

A catheter connector allows medical staff to supply a range of fluids to ill patients using a single intravenous line. In the case of premature babies, such a line provides nutrition in the form of a synthetic milk (TPN) and antibiotics to prevent infection. The connectors are made from several alternative polymers, but polycarbonate is used widely. However, the material is susceptible to environmental stress cracking (ESC) from a variety of fluids. The case study shows how the first versions of one design were poorly moulded and cracked during service, allowing bacteria to enter the line and so infect the baby. The baby contracted meningitis, but lived although suffering brain damage. Evidence was produced which showed that it was a widespread problem in the mid-1990s in UK hospitals. The investigation suggested that poor design and manufacture were the root cause of the problem.     

Environmental stress cracking of glassy polymers and solubility parameters

The use of solubility parameters to predict critical stress ( _c ^* ) or strains (_c) for environmental cracking/crazing in several glassy polymers (e.g. PMMA, PPO, PS, PVC, PSF and PC) is re-examined. It is shown that the enthalpic component ( _H ) of the Flory-Huggins semi-interaction parameter () does not always give a good correlation between _c and _H even though solvent molar volume and polymer-solvent molecular interactions have already been considered. Re-analysis of available experimental data using Gent's theory shows that there is a general trend for _c ^* (or _c) to increase with . These results, therefore, support Gent's proposed mechanism of environmental stress crazing/cracking. It is finally concluded that unless a definite relationship can be established between _c or _c ^* with _H it is not possible to predicta priori _c or _c ^* , given the empirical solubility parameters of a solvent. Unfortunately, there are not many such relationships as discovered in this paper.     

Environmental stress cracking behaviour of urethane methacrylate based resins

Two kinds of cross-linked urethane methacrylate resins have been investigated using three-point bend tests to determine their environmental stress cracking (ESC) behaviour in a range of liquids (water, sodium hydroxide, ethylene glycol, acetonitrile, acetic acid, acetone, tetrahydrofuran, toluene, 3,5,5-trimethylhexanol and petrol). The resins were found to undergo ESC in organic liquids only, and the critical strains, _c, and critical stresses, _c, have been related to the solubility parameters, , of the liquid environments. The most severe ESC was observed in solvents with –19–20 MPa^1/2, corresponding to minimum points in the plots of _c and _c against . Generally, the resin with the higher cross-link density had a greater resistance to ESC, but the effect of liquid diffusion complicated the situation and was found to play an important role in the ESC behaviour of these materials. The results confirmed that liquid diffusion into the resins lowered the critical strain (and stress), leading to earlier failure. In the case of the lower cross-link density resin, very fast diffusion was found to cause softening. However, it was noted that liquid diffusion can also blunt crazes and cracks.     

Effective stress intensities in stress corrosion cracking

The phenomena of branching and blunting of stress corrosion cracks are reviewed and their effects demonstrated for a martensitic steel. The stress intensity that a crack can sustain is proportional to the square root of its tip radius, so that blunt cracks require a higher apparent stress intensity. A simple procedure is outlined for converting apparent stress intensities to effective stress intensities, so eliminating anomalous effects due to crack branching and blunting.

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Résumé On passe en revue les phénomènes de ramification et d'arrondissement de l'extrémité des fissures de corrosion sous tension, et on examine leurs effets dans le cas d'un acier martensitique.L'intensité de contrainte qu'une fissure peut supporter est proportionnelle à la racine carrée de son rayon d'entaille, en sorte qu'à des fissures émoussées correspondent une intensité apparente de contrainte plus élevée. On propose une procédure simple permettant de convertir les intensités apparentes de contraintes ou intensité effective, ce qui permet d'éliminer les effets parasites associés à la ramification et à l'arrondissement des fissures.

     

Chemical stress cracking of acrylic fibres

The generation of periodic microscopic transverse cracks in oriented acrylic fibres immersed in hot alkaline hypochlorite solution is described in detail and shown to be a variety of chemical stress cracking. It is greatly accelerated by external tensile stress, high fibre permeability, moderate fibre orientation, and water-plasticization. The proposed mechanism for bond cleavage involves cyclization of nitrile groups (similar to the prefatory reaction in pyrolysis of acrylic fibres), followed immediately by N-chlorination and chain scission. Mechanical retractile forces (internal or external) then cause chain retraction and crack growth. Despite the remarkable regularity of the crack pattern, which typically resembles a series of stacked lamellae, the process is independent of any such underlying fibre morphology. The cracking process does, however, appear to be a sensitive indicator of residual latent strain in the fibre, which may persist even after high-temperature annealing.     

Initiation of stress corrosion cracking and fatigue crack under compressive stress

Macroscopic compressive stress was able to induce stress corrosion cracking (SSC) of type 304 stainless steel in a boiling 42% MgCl₂ solution and of mild steel in a boiling 60% Ca(NO₃)₂ + 3% NH₄NO₃. The incubation period of SCC under the compressive stress was ten to hundred times longer than that under the tensile stress.For ultra-high strength steel and aluminum alloy, a fatigue crack could initiate from a notch tip under a cyclic compressive load. The threshold value Δσ_th or Δ$\text{K}{}_{\mathrm{th(\rho )}}$ for fatigue crack initiation under the compressive load was four times as high as that under tensile load. The crack grew at a decreasing rate until eventually it stopped growing altogether under cyclic compressive load. The crack nucleation under compressive stress became easier and the propagation distance of the fatigue crack was longer if the minimum cyclic compressive load was near zero.     

Hydrogen-assisted stress cracking of high-strength wheel bolts

Early failures occurred with two sizes of wheel bolts used for attaching front wheels and dual rear wheels to heavy truck hubs. Failure resulted in fracture of the bolts, and was a response to the material and process specification which produced a steel microstructure highly susceptible to hydrogen-assisted stress cracking. The microstructure resulted from an alloy steel that was carburized, heat treated, and then zinc plated. This combination of material and processing produced a high-strength SAE Grade 8 bolt (equivalent to an ISO 10.9 grade) with a hard, brittle case and an anodic zinc coating. A slight misalignment of the wheel bolt coupled with a ball seat mounting design for the wheel nuts created a combined axial and bending stress that exceeded the threshold for hydrogen-assisted stress cracking.     

PET瓶应力开裂原因的探讨及预防

用于灌装碳酸饮料的PET瓶经常会遇到应力开裂问题,文中从高分子物理角度进行开裂机理的分析,探讨关键性的根本因素-原料对应力开裂的影响,以及针对应力开裂问题所采取的预防措施.     

The control of catalytic poisoning and stress corrosion cracking

When a structural metal is stressed in a hydrogen environment, the metal may crack at stress levels much lower than its normal strength. This embrittlement is caused by hydrogen atoms or ions rather than hydrogen molecules. The catalytic dissociation of hydrogen molecules into atoms or ions takes place at dislocation sites on a metal surface. The termini of dislocations on a metal surface are sites of localized high energy. There are “poisons” which will retard or even stop catalytic processes. The toxicity of a poison depends on the binding energy of the foreign atom to a dislocation terminus.Cracked specimens made of 4340 steel were tested in hydrogen gas and its mixture with SO₂, CS₂, CO₂, N₂ and Ar. Both SO₂ and CS₂ are very “toxic” and can stop a running crack. The toxicity of CO₂ is moderate. n₂ and Ar have no noticeable effect on a running crack.     

Residual stress effects in sharp contact cracking

A detailed strength analysis for brittle surfaces containing dominant flaws produced under elastic-plastic indentation loading is presented. The condition for failure is formulated in terms of stress intensity factors representing driving forces associated with applied tension and residual indentation fields. Incorporation of the second of these components depresses the equilibrium applied stress-crack size function; this depression is accentuated at small crack size, such that the function passes through a maximum. Depending on the relative intensity of the residual indentation field, the starting size of the median cracks, as determined from Part 1 of this study, may lie on either side of this maximum: large cracks, i.e. those starting beyond the maximum, fail spontaneously from an unstable branch of the applied stress curve; small cracks undergo precursor stable growth to a critical depth at the stress maximum before failing. Observations of median crack growth in annealed and tempered soda-lime glass discs taken to failure in biaxial flexure confirm the existence of an energy barrier to crack instability. The important implications of these manifestations of the residual indentation field in predicting strength degradation characteristics for prospective adverse contact conditions are discussed for test pieces subjected to various imposed surface stress states.     

Residual stress driven cracking in superalloy weldments

Superalloy weldments are normally given post weld heat treatments to homogenize the weld metal microstructure, relieve residual stress, and precipitate strengthening phases. The relationship between microstructure and post weld heat treatment is easily studied; it is less straightforward to study the effects of post weld heat treatment on residual stress relaxation. Using a self-restrained testing procedure, a relatively simple approach was used to investigate the effects of microstructure and post-weld heat treatment on cracking during residual stress relaxation. Candidate superalloys for Advanced Ultra Supercritical steam plants were studied. It was found that cracking due to residual stress relaxation is primarily dependent on grain size, and in cases of intermediate grain size, intragranular precipitation is a controlling factor. These results are in agreement with traditional stress relaxation cracking theories.